Electronic tuning device



Patented Feb. 5, 1952 ELECTRONIC TUNING DEVICE Thomas H. Long, Elkhart, Ind., assignor to C. G. Conn, Ltd., Elkhart, Ind., a corporation of Indiana Application October 8, 1948, Serial No. 53,505

11 Claims. (Cl. 250-36) My'invention relates to an improved mechanism for producing oscillations suitable for tunin musical instruments.

When musical instruments, such as the string, woodwind and brass instruments of an orchestra,

are tuned, it is desirable to have available a source of sound which is accurately calibrated to either A (440 cycles) or B flat. Moreover, to accommodate the tuner to desired variations from the standard frequencies of these two notes, it is desirable to provide an accurately calibrated linear scale and associated adjusting mechanism for varying the frequency of the notes over a limited, accurately precalibrated range.

In addition, it is desirable in a tuner intended for various types of instruments to have a selectable tone that may be' chosen in accordance with the particular instrument being tuned, thus making possible a more accurate ear comparison of the relative pitches of the two tones than could otherwise be made.

It is therefore a general object of the present invention to provide an improved device to produce tones suitable for tuningmusical instruments.

A more specific object of the present invention is to provide an improved device to generate tones suitable for tuning musical instruments and which is simple and inexpensive in construction while at the same time may be accurately calibrated and is capable of limited accurately calibrated variations from the normal reference frequencies. I

Yet another object of the present invention is to provide an improved device to produce tones for tuning musical instruments and which is se- -'lectively operable to produce either of two accurately calibrated reference tones or accurately preselected variations therefrom.

- to provide an improved source of energy for driving the rotating disks of a stroboscope type frequency measuring device and in which an adjustable oscillator is substituted for an adjustable tuninsfork.

Another object of the present invention is to provide an improved source of energy for driving the rotating disks of a stroboscope type frequency measuring device and which may be accurately calibrated by one-point tests using a f standard tuning bar of fixed frequency.

Yet another object of the present invention is to provide an improved oscillator wherein changes in applied voltage do not alter the frequency of oscillation.

The novel features which I believe to be cha acteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of one form of the invention and Figure 2 is a circuit diagram of another form thereof.

Referring now to Figure 1 there. are shown at H! and 12 a pair of electron tubes, tube l0 being a triode, such as one set of elements in a 12AU7 tube, and tube l2 being a pentode, such as a B5. Circuit connections with tube In define an oscillator whereas the circuits associated with tube l2 form an amplifier for the oscillations produced thereby. The amplified oscillations are applied to loud speaker [4 to generate tuning tones.

The inductor indicated generally at I6 has a main winding Ifia with a total of five spaced taps, I61), I60, 16d, IGe and 16f. This winding, or a preselected portion thereof, is connected through switch [8 and potentiometer 20 to the capacitor 22 to define a resonant circuit having a reactance element (winding l6a) with five spaced taps.

Tube I0 is connected to the resonant circuit defined by inductance I 6 and capacitor 22 to form a Hartley type oscillator circuit. To this end, the control electrode of tube It] is connected to tap 16b or tap I by the circuit through resistance 24, capacitor 26, and switch I8; the anode of tube I0 is connected to tap I66 and tap l6) through potentiometer 20; and the cathode of tube I9 is connected to tap l6d through resistance 58 and capacitor 54. I

Resistance 32 acts as a grid leak to maintain control electrode-cathode bias voltage on tube It].

From the foregoing description it will be evident that the tube III is connected in an oscillator circuit in which the frequency of oscillation is determined by the resonant frequency of the resonant circuit defined by inductor I60. and capacitor 22 and hence the value of the inductor Hie.

The taps H512 and We are so positioned that the tube In oscillates at one preselected frequency, say A? (440 cycles), when switch is is at the position shown in Figure l and at another preselected frequency, say B flat," when the switch is in the other position.

An alternative method of obtaining a precise semitone interval from A to B flat is that of adding a sixth tap to winding Mia. The turns between this sixth tap and IE1) or I60 would be about one-quarter of one percent of the total turns of winding Ilia. Resistance 34 would be replaced by a second potentiometer like potentiometer and connected to the additional tap and to the cooperating tap I6b or IBc and to the corresponding terminal of switch I8.

One of the features of the present invention resides in the fact that the frequency interval between the two positions of switch I8 is accurately fixed without precisely fixing the position of the taps relative to each other. This is-accomplished by the variable resistance 34 which is connected between switch I8 and tap I61). I have discovered that varying the value of this resistance alters the frequency of the oscillations to a slight extent and permits compensation for the errors inherently associated with inaccuracies and-variations in the Winding Mia and the taps provided thereon.

The potentiometer 20 acts as a Vernier tone adjusting device and permits preselected calibrated pitch changes over a small range of frequency. I

have discovered that the pitch variation thus obtained is quite linear with movement of contact 20a and that the pitch variation from the center or normal frequency for particular movements of the contact is substantially the same regardless of the position of switch I8.

Further in accordance with the present invention, the resistance value of potentiometer 20 is chosen at a value introducing minimum disturbance to the oscillation of tube I0. this resistance is substantially the geometric mean value of the maximum resistance at which oscillations occur at all positions of potentiometer 20 and the minimum resistance at which oscillations occur on all positions of potentiometer 20. Thus, if the former value is 5600 ohms, and the latter value 86 ohms, the resistance of the potentiometer may be chosen at 500 ohms which is sufiiciently close to this geometric mean value for effective operation.

In addition to winding IGa, the inductor I6 has an auxiliary or output winding I69. The latter winding is magnetically coupled to Winding I6a and is connected to one terminal of switch 35.

The moving terminal of this switch is .con-

nected to potentiometer 36 which on its opposite end is connected back to winding l6g.

The other terminal of switch 35 is connected to the common connection of capacitor and resistance 28.

Switch acts as a tone selector to produce 05- cillations across potentiometer 36 havingeither of two tones. When this switch is in the position shown in Figure l, the tone is nearly of sine wave form and simulates the tone of a flute. When switch 35 is in the other position the tone is richer and corresponds more nearly to a reed instrument. Thus either tone may be selected in accord with the instrument being tuned to achieve most effective tuning.

The effect of resistance 28 is to preload the os cillator I0 when switch 35 is adjusted to the reed position. This prevents the pitch varia- Specifically,

tions that would otherwise be associated with vol- The tube I2 operates as an amplifier to apply oscillations from oscillator ID to the speaker I4. The function of resistor 40 and capacitor 42 is to provide cathode-control electrode bias for this tube. Transformer 44 feeds the alternating component of the cathode-anode space current wave .of this tube to the speaker I4 and, in addition,

matches the impedance of speaker I4 to the out put impedance of tube I2.

The unit of Figure l is energized from source 45 which may, for example. be a source of 110 volt, 60 cycle, alternating current. When switch 48 is closed, the heaters Illa and I2a of tubes III and I2 are energized through resistance 50. In addition, cathode-anode space path voltage is applied to these tubes by rectifier 52 and the filter defined by capacitors 54 and the field coil Ila of speaker I4. Resistance 56 serves to provide a reed tone signal and render the tone frequency relatively independent of tubecharacteristics, applied voltage, etc.

In a practical tone tuning device embodying the circuit of Figure 1, the following component elements may be used:

Tube I0 one set of elements of a. 12AU7 type tube.

Tube I2 B5 type tube.

Resistance 32 470,000 ohms Resistance 24 470,000 ohms Resistance 28 47,000 ohms Resistance 34 300 ohms Resistance 38 220,000 ohms Resistance 40 150 ohms Resistance 50 370 ohms Resistance 56 10,000 ohms Potentiometer 20 500 ohms Potentiometer 36 500,000 ohms Capacitor 26 0.01 mfd.

Capacitor 22 0.03 mfd.

Capacitor 30 0.003 mfd.

Capacitorv 42 8. mid.

Capacitor 54 40. mid.

Transformer I8 Winding I69 1'75 turns Winding IBa Between taps I6f and I6e '73 turns Between taps Se and ML. 1217 turns Between taps Mid and l6c 1252 turns Between taps I50 and UL. 146 turns The inductance of reactor I5 is adjusted by varying the air gap or by similar means until the oscillation frequency with potentiometer 20 at midposition and switch I8 connected to tap IE0 is B flat (466.16 cycles per second). Switch 34 is then turned to contact resistance 34 and the value of that resistance adjusted until the oscillation frequency is A (440 cycles per second).

The modified structure of Figure 2 is intended to generate alternating waves suitable for driving a synchronous motor for a stroboscopic frequency measuring device. In this structure, the elements of section 60a of tube 60 are connected to inductor 52 to define an oscillator circuit of the Hartley type. This inductor has four spaced taps, 62a, 82b, 62c and 62d. Tap 62a is connected to the control electrode of section 60a of tube 60 by capacitor 54 and resistance 66. Tap 62b is connected to the cathode by capacitor 68.

The anode of section 50a of tube 60 is variably connected to taps 62c and 62d by potentiometer I0 and the ganged pair of resistors I2 and 12a.

Resistors I2 and 12a cooperate to provide a vernier frequency adjustment by means of ,which frequency of the oscillations.

small variations of frequency can be made without altering the scale calibration of the principal frequency control 10. It is thus possible to corpoint calibration with a fixed frequency tuning bar. This does not alter the scale of control 10' and hence effectively calibrates the unit for op- 'eration over the entire useful range of operation.

Potentiometer 10 is provided with an appropriate dial and scale. Preferably this is accomplished by the use of a preprinted scale and tolerance variations in the total resistance value of controls l0, l2 and 12a are compensated by v resistance 13 which is adjusted at the factory.

The resistors 12 and 12a are mechanically connected to be moved simultaneously by a common adjusting element such as a knob. However, they are interconnected in opposite senses so that the value of one resistance is increased as the value amplifier indicated generally at 14. The latter furnishes driving power for the motor 16 which drives a marked disk at speed determined by the This disk, when illuminated by timed light flashes, permits accurate frequency measurements. A systein-of this type is described in R. W. Young Patent 2,286,030, assigned to the same assignee as the present invention.

When the tuning mechanism is calibrated in the field, the stroboscopic pattern is controlled by a fixed frequency tuning bar and control 10 set at the scale position corresponding to the frequency of the bar. Controls 12 and 12a is then charge tube 69 cooperates with resistance 69a and resistance 84 to reduce voltage variations on the oscillator circuit to an amount just com pensating for heater current changes, thus -pre-.

venting line voltage changes from influencing oscillator frequency. voltage for amplifier 14 and tube 60 is derived from terminals 18 and 80 to which a source of D.-C. potential is connected. Resistances 82 and 84 and the capacitor 86 serve to reduce thisvoltage for tube 60 and to isolate the voltages applied to the various elements therein.

The terminals I8 and 80 are energized from rectifier It!!! by the alternating voltage source I02. The heaters of tubes 60 and 14 are likewise energized from source H12 but through transformer I04. If for any reason the voltage of source I02 should vary, the voltage applied to the heaters of tube 60 would likewise vary and thus tend to alter the frequency of oscillation of section 60a of that tube. ever, the cathode-anode space path voltage applied to section 60a of tube 60 is altered by reason of the variation of the output voltage of rectifier I00 and the action of gaseous discharge tube 69.

This variation tends also to alter the frequency Cathode-anode space path Simultaneously, howof oscillation but in opposite sense of the variation associated with the changed heater current. The values of resistances 69a and 84 are so chosen that these effects compensate with the result that changes in voltage of source I02 do not influence the frequency of oscillation.

Resistances 88 and 90 act as a voltage divider to feed to section 60b of tube 60 a portionof the voltage produced by section 60a. Resistance 92 and capacitor 94 act to produce a cathode con-'- trol electrode bias voltage for section 60bv of tube 60.

Amplifier 14 is of conventional construction embodying a pair of push-pull 'triode tubes and input and output transformers. The latter is connected to the synchronous motor 16.

In both of the diagrams the oscillator plate is shown connected to that side of the tank circuit that includes the potentiometer arrangement. Either of these circuits will operate equally well with the leads from the tankcircuit to the oscillator plate and grid blocking capacitor interchanged.

From the foregoing description it will be evident that I have invented an improved tone generator requiring only standard component elements with the usual tolerances but which is nevertheless capable of producing accurately preselected frequencies which may be varied over a desired tuning range. Moreover, either of two tones and two sounds may be reproduced as desired.

In the appended claims I have referred to the tube electrode to which the movable contact 20a of the resistance 20 is'connected as a terminal" which is selectively connectable to taps l6e and I6! to cause oscillator operation at either of two frequencies. The similar terminal in the structure of Figure 2 is the anode 60a of the tube 60.

While I have shown and described two specific embodiments of my invention, it will, of course, be understood that I do not wish to be limited thereto but intend by the appended claims to cover all variations and alternative constructions as fall within the true spirit and scope of my invention.

. What I claim as new and desire to secure by .Letters Patent of the United States is:

1. In an oscillator of the type having a tuned circuit with a reactance element having spaced taps selectively connectable to a terminal to adjust the oscillator for operation on either of two frequencies, the improvement which comprises a each end of said resistance and said taps.

3. In an oscillator of the type having a tuned circuit'with a reactance element having'spaced taps selectively connectable to a terminal to ad'- just the oscillator for operation on either of two frequencies, the improvement which comprises a resistance connected between said taps and having a movable contact member, and means conselectively connectable to a terminal to adjust the oscillator for operation on either of two frequencies, the improvement which comprises a variable resistance interposed between one tap and the terminal. r

5. Incombination, an electron tube having cathode, anode, and control electrodes, a resonant circuit having a reactance element with four spaced taps, means connecting one end tap to said control electrode, means connecting the adjacent tap to said cathode, a resistance connected between the other two of said taps, and a movable terminal on said resistance connected to said anode.

6.111 combination, an electron tube having cathode, anode, and control electrodes, a resonant circuit having a reactance element with four spaced taps, means connecting one end tap to said control electrode, means connecting the adjacent tap to said cathode, a resistance connected between the other two of said taps, and a movable terminal on said resistance connected to said anode, the value of said resistance being substantially the geometric mean value between the maximum value at which oscillations are achieved on all settings of said movable terminal and the minimum value at which oscillations are achieved on all'settings of said movable terminal.

7. In combination, an electron tube having cathode, anode, and control electrodes, a resonant circuit having a reactance element with five spaced taps, means connecting the center tap to said cathode, and means connecting said anode and said control electrode each to one of the end taps respectively, said last means including a potentiometer connected to a pair of taps including one end tap and the adjacent tap and having its movable contact connected to one of the electrodes of said tube, and a switch havirn terminals connected to another pair of taps including the other end tap and the adjacent tap and a switchable contact member connected to another of the electrodes of said tube.

8. In combination, an electron tube having cathode, anode, and control electrodes, a resonant circuit having a reactance element with five spaced taps, means connecting the center tap to said cathode, and means connecting said anode "and said control electrode each to one of the end taps respectively, said last means including 'a'potentiometer connected to a pair of taps including one end tap and the adjacent tap and having its movable contact connected to one of the electrodes of said tube, a switch having terminals connected to another pair of taps including the other end tap and the adjacent tap, a moving contact member on said switch connected to another of the electrodes of said tube, and a resistance interposed between one of said switch terminals and the corresponding tap.-

9. In a-device to generate oscillations suitable fortuning musical instruments, an electron tube having cathode, anode and control electrodes. 8. resonantcircuit having a reactance element with nve spaced taps, means connecting the center tap to said cathode, switch means selectively operable to connect either end tap and tap adjacent thereto to said control electrode, a variable resistance interposed between said switch means and the end tap to which it is connected, a potentiometer connected to the other end tap and tap adjacent thereto and having its movable contact connected to said anode, an output circuit for the device including a potentiometer connected to said resonant circuit, an electron tube having cathode and control electrodes, means connecting one end of said last potentiometer to said cathode and the movable contact thereof to said control electrode, and a resistance interposed between said last movable contact and said last control electrode.

10. In an oscillator having an inductor in which oscillations are produced and adapted to energize an output circuit of variable impedance, a winding inductively coupled to said inductor, switch means selectively operable to connect said winding to said output circuit or a point on said inductor to said output circuit, whereby variations in impedance of said output circuit vary the frequency of oscillation when said inductor is connected thereto, and a resistance connected across a portion of said inductor to minimize said variations.

11. In an oscillator of the type having a vacuum tube having cathode, anode, and control electrodes connected to a reactance element to produce oscillations, the improvement which comprises a pair of spaced taps at one end or said reactance element, a potentiometer connected to said taps and having its movable element connected to one of said electrodes, said potentiometer having a calibrated dial, resistances interposed between said potentiometer and said taps and simultaneously adjustable in opposite sense, and an additional adjustable resistance interposed between one side of said potentiometer and the corresponding tap to compensate for tolerances in the value of said potentiometer and said first resistances.

- THOMAS H. LONG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 906,211 Ferrie Dec. 8, 1908 1,530,498 Kendall Mar. 24, 1925 2,043,828 Coupleux June 9, 1936 2,045,172 Yungblut June 23, 1936 2,211,540 George Aug. 13, 1940 2,440,284 Levy Apr. 27, 1948 2,441,334 Sayer May 11, 1948 FOREIGN PATENTS Number Country Date 338,510 Germany June 1, 1921 

